The invention relates to a process or a device for determining the distance between a distance sensor and an object.
Processes for distance measuring in accordance with the propagation time principle are already known.
In the case of a known propagation time measuring process, a light pulse is emitted at a predetermined starting time and, simultaneously, a counter is tripped. The light pulse is reflected on an object whose distance is to be determined and the reflected light pulse is picked up by a receiver and is verified by means of analog detection electronics. At the moment of verification, the counter is stopped and from the elapsed Time duration as well as the known speed of light (c=300,000 km/s), the distance of the object is determined. Due to the high speed of the light, rapid detection electronics are required.
One task of the present invention lies in indicating a distance-determining process which provides a high degree of measuring precision while requiring a minimum of hardware.
Furthermore, one task of the present invention consists in the creation of a device that determines the distance with a high degree of measuring precision and that can be produced in a cost-efficient manner.
Through direct scanning of the incoming signal by means of an A-D converter, a cost-efficient arrangement can he realized since detection electronics for monitoring the analog incoming signal are not required. Instead, the incoming signal is digitalized directly, i.e. prior to its evaluation, whereby it is made possible to perform all additional signal processing and evaluation by computer.
However, the digitalization of the incoming signal has as a consequence the temporal resolution of the digitalized signal being limited by the scanning frequency (f2) of the A-D converter. For example, with a scanning frequency of f2=40 MHz and v=c, an individual scanning interval corresponds to a path of 3.75 m. In accordance with the process of the invention, the distance resolution is increased in that not only one but several--namely k wave pulse propagations time values (T1, T2, . . . 1 Tk) are determined and from these k propagation time values, the arithmetic mean (T) is calculated. This averaging increases the distance resolution since, due to the A-D converter, the [scanning] instants are independent with respect to time from the emission instants of the individual wave pulses. The asynchronous scanning makes it possible for, with the pulse propagation times of different wave pulses which are actually identical (i.e. in the case of a continuous distance), a stochastic distribution of the measured propagation times (T1, T2, . . . , Tk) to be registered. Due to averaging in accordance with the invention via the k measured propagation times (T1, T2, . . . , Tk), which are distributed stochastically at the actual propagation time, the resolution of the measurement is then increased by the factor k. Due to this increase of the measuring resolution, created with the aid of a software solution, on the other hand, a realization of the process of the invention is made possible which is more cost-efficient with respect to the hardware requirements. The cause for this can be seen in the fact that, due to the increase in resolution for the purpose of achieving a predetermined measuring resolution of the total system required in practice as achieved in accordance with the invention through calculation, it is possible to use an appropriately slower and, hence, more cost-efficient A-D converter instead.
While basically, for the purpose of determining each individual wave pulse propagation time (Ti), i=1, . . . , k, respectively, merely the digital signal (Z(t) obtained with respect to a received wave pulse has to be evaluated, the determination of the individual wave pulse propagation times, Ti, i=1, . . . , k, in accordance with a preferred embodiment of the invention, takes place in accordance with a mean digital signal &lt;Z (t)&gt; obtained on the basis of n emitted wave pulses. The signal averaging via n signal sequences, which has nothing to do with the previously described time averaging via k-calculated pulse propagation times, causes the mean digital signal &lt;Z(t)&gt; to have a substantially improved signal-to-noise ratio, since noise components are lost during averaging while signal components add up. Therefore, by increasing n, it is possible to increase in particular the measuring range of the distance measuring process of the invention.
However, it must be taken into consideration that the measuring time increases proportionately with respect to n. Therefore, it is advantageous to always select n great enough in order to ensure that the signal-to-noise ratio of the mean digital signal &lt;Z(t)&gt; is just sufficient for additional signal processing.
Preferably, n is selected between 10 and 100. Since the improvement of the signal-to-noise ratio is proportional to n, a 3.16- to 10-fold increase of the signal-to-noise ratio is thereby achieved.
An additional advantageous variation of an embodiment of the present invention is characterized in that the evaluation of the digital signal Z(t)--or, in the case of signal averaging, the evaluation of the mean digital signal &lt;Z(t)&gt;--respectively comprises the following steps for the calculation of the individual pulse propagation times Ti, i=1, . . . , k:
Calculation of a correlation function F (.tau.)=in accordance with the equation ##EQU1##
wherein R(t) is a predetermined, digitalized reference signal with a temporal resolution which is higher by an order of m than Z(t) or &lt;Z(t)&gt; and wherein m is an integral number with m&gt;1, and determination of the wave pulse propagations time T1 (and during subsequent passes, also of the remaining wave pulse propagation times T2, . . . Tk) as that time at which the correlation function F(.tau.) is maximum.
On the one hand, the correlation serves to improve in an appropriate manner the temporal resolution of the received digital signal Z(t) or &lt;Z(t)&gt;--possibly averaged via n wave pulses--by convolution with a reference signal R(t) having a temporal resolution which is higher by an order of m. For this reason, it is possible to improve the distance resolution of the measuring process of the invention by increasing m in a manner similar to that of increasing k. On the other hand, the correlation is used for determining the wave pulse propagation times Ti, i=1, . . . , k by means of a maximum value analysis of the function F(.tau.).
Preferably, m lies between 2 and 10. When selecting m, one must take into consideration that as m increases, the complexity of the calculation increases.
Preferably, from the mean digital signal &lt;Z(t)&gt;, the reflected total intensity and/or the maximum intensity and/or the signal-to-noise ratio is determined and at least one of these parameters is used for controlling the amplification of an amplifier amplifying the incoming signal. For example, the control may take place in such a way that the amplification is increased when the reflected total intensity or also the maximum intensity drops below a predetermined value.
In a preferred manner, from several determined distance values T, data may also be obtained via a relative movement between the object and the sensor. This takes place in such a way that the obtained distance values (D) are entered in a memory and from several distance values (D) the continuous relative speed between the object and the distance sensor is determined by differentiation with respect to time.
Basically, the sensor may be of any desired type. A highly efficient embodiment of the device of the invention, particularly applicable in motor vehicles, is characterized in that the sensor comprises an infrared laser diode as an emitter.
Advantageously, after the A-D converter, an intermediate memory is provided which continuously stores the digital signal and from which the evaluation circuit receives the digital signal by reading the intermediate memory. For this purpose, a FIFO method is particularly suited because its read-in cycle is independent of its output cycle and, for this reason, it is suited for the coupling of asynchronous systems.